The need for materials for cutting tool applications, with improved toughness, good strength at elevated temperatures and chemical inertness, and capable of operating at high cutting speeds has generated a widespread interest in ceramic materials as candidates to fulfill these requirements. Conventional ceramic cutting too materials have failed to find wide application primarily due to their low fracture toughness.
Therefore, many materials have been evaluated to improve ceramic performance, such as silicon nitride based composite for cutting tool applications. Specific examples of silicon nitride based composite cutting tools are discussed in U.S. Pat. No. 4,388,085 to Sarin et al. (composite silicon nitride cutting tools containing particles of TiC); U.S. Pat. No. 4,425,141 to Buljan et al. (a composite modified silicon aluminum oxynitride cutting tool containing particulate refractory metal carbides, nitrides, and carbonitrides); U.S. Pat. No. 4,433,979 to Sarin et al. (composite silicon nitride cutting tools containing particulate hard refractory carbides or nitrides of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W); and U.S. Pat. No. 4,449,989 to Sarin et al. (composite silicon nitride cutting tools coated with two or more adherent layers of refractory materials).
Many improvements have been made in the toughness, abrasion resistance, high temperature strength and chemical inertness, but increased demands by the cutting tool industry require cutting tools with new and improved characteristics. In many applications, for example in gray cast iron and high nickel alloy machining, silicon nitride tool wear has been found to be dominated by abrasion. Even at cutting speeds as high as 5000 sfm, chemical reactions between tool and workpiece are negligible in comparison. It has been found that abrasion resistance .for silicon nitride ceramic cutting tool materials is directly proportional to K.sub.IC.sup.3/4 H.sup.1/2, where K.sub.IC is the fracture toughness and H is the hardness. It may be seen, therefore, that further improvement in the fracture toughness of silicon nitride ceramic materials could bring about significant increases in both reliability and abrasive wear resistance, providing materials for cutting tools with new and improved characteristics. The present invention provides such new and improved ceramic materials.
The wear-resistant composite materials of the invention are also expected to find wide use in wear part and structural applications, for example in dies, turbines, nozzles, etc.